The field of the invention is small engines and systems for controlling the emissions from such engines.
Various methods are used to control the emission of detrimental engine exhaust constituents such as nitrogen oxides, carbon monoxide and hydrocarbons. One method employs an oxygen sensor in the engine exhaust to measure the oxygen level and develop a control signal which operates a valve that effects the air/fuel ratio input to the engine. If the air/fuel mixture is too rich, excessive carbon monoxide and hydrocarbons will be emitted and the exhaust oxygen will be low. On the other hand, if the air/fuel mixture is too lean, engine power will be reduced.
Many systems have been developed for controlling the air/fuel ratio as a function of exhaust gases. Most have been developed for use on automobile engines which have on-board computers that control many functions. Typically, the analog signal produced by the oxygen sensor is digitized and input to the computer. The computer performs a number of calculations and outputs a digital control signal which is converted to analog form and applied to a motor which controls a carburetor or fuel injector valve.
While many highly sophisticated systems have been developed for automobiles, these are not suitable for use on small engines. It can be appreciated that the initial cost of an automobile engine and the maintenance which it receives is vastly different than a lawn mower engine. For economic reasons it is not possible to use digital computers and the associated support circuitry on small engines. Accordingly, purely analog circuits have been employed on small engines and because of cost constraints and the sensitivity of analog solutions to temperature and age, these analog circuits are very limited in their capability.